Abstract:

At 2:04 on May 22, 2021, an earthquake of M7.4 occurred in Maduo County, Golog Prefecture, Qinghai Province, with the focal depth of 17 kilometers, the epicenter at 34.59°N and 98.34°E. This earthquake was the largest after the Wenchuan earthquake in China. The epicenter of the earthquake is 38km away from Maduo county seat and 385km from Xining, the provincial capital. The earthquake caused some houses to collapse and some damage to roads in the epicenter. But due to the sparse population in the epicenter area, the earthquake did not cause casualties.

Seismologist believe that the earthquake is the result of the continuous activity of the boundary fault of the Bayankala block, which is geographically located in the north of the Qinghai-Tibet Plateau and is the hub for the transformation of the direction of the crustal movement of the plateau. In recent years, many destructive earthquakes occurred inside the block. This earthquake is another strong earthquake after the M7.1 Yushu earthquake in Qinghai in 2010. According to the analysis of this earthquake briefing, the fault zone that induced this earthquake is speculated to be the Maduo-Gande fault zone or the Kunlun Mountains Pass-Jiangcuo fault zone.

In order to find out which fault is the seismogenic structure and the distribution of the seismogenic structure of this earthquake, we relocated the dense earthquakes by double-difference method based on the data of 1357 aftershocks in the Maduo M7.4 earthquake area recorded by 72 fixed stations of the digital seismic network of Gansu and its adjacent seismic network and 12 portable seismographic stations during the May 22 to May 27, and obtained the source parameters for 1289 earthquakes. The accurately located small earthquakes distribute along both sides of the Kunlun Mountains Pass-Jiangcuo Fault, which is NNW-trending obviously. It shows that the seismogenic structure of this earthquake is the Kunlun Mountains Pass-Jiangcuo Fault, rather than the Maduo Gande Fault as considered previously by some scholars. This is consistent with the research results of surface fracture zone, magnetotelluric detection, InSAR coseismic deformation and relocation of other aftershocks. Most earthquakes distribute at the depth range of 0~15km of the crust after the relocation, and the result shows that the focal depths are more concentrated. The relocation also shows that the east and west ends of the main fault have bifurcations. It may be that the complex stress distribution triggered two new branch faults during the occurrence of the great earthquake, and the overall fault shows a “tree-type” structure. The west branch trends 306°and intersects the main fault at 21°. The east branch is nearly EW trending and connected with the east section of the main fault.

Generally, the earthquakes are closely related to active tectonics, large earthquakes and its aftershocks usually occur on fault zones with obvious activity. The distribution of small earthquakes is related to the complex underground stress state and the complex structure of the fault zone. We can inverse the shapes and positions of the fault planes using spatial distribution of hypocenters of mainshock and the corresponding aftershocks, according to the principle that clustered earthquakes occur near the faults. Six rectangular regions are selected according to the distribution characteristics of relocated aftershocks and by reference to the distribution of geological faults and earthquake rupture zones. We obtained the detailed parameters of fault plane in each region by using the simulated annealing algorithm and the Gauss-Newton algorithm according to the source information after the relocation in 6 rectangular areas. On this condition, rake angle of the fault plane is further inferred from regional tectonic stress parameters. The results show that the main fault is a large, high dip angle, sinistral strike-slip fault with thrust component, striking 285°~290° and about 146km long. It extends from Tanggema Township of Maduo in the southeast(34.49°N, 98.91°E)to Gazejialong Township in the northwest(34.81°N, 97.54°E). The movement characteristics of the newly generated western segment 2 show dextral strike slip and thrust, which is diametrically opposite to that of the main fault. This shows the complexity of the earthquake rupture process, and further research is needed on the tectonic mechanics and deep structures that produce this special rupture.

Compared with the focal mechanism solutions obtained by domestic and foreign authorities, the fault plane parameters obtained in this paper are similar to them, indicating that our conclusions are reliable. Besides, the spatial distribution of inverted fault plane is basically identical to that of the rupture zone derived from post-earthquake investigation in the earthquake area.

Key words: double-difference location approach, the Maduo M7.4 earthquake, fault plane solution, seismogenic structures

摘要：

文中基于青海和周边地震台网72个台站以及震后布设的12个流动观测台站于2021年5月22-27日记录的青海玛多M7.4地震主震及1 357次余震资料, 使用双差地震定位法重新对余震位置进行了修定, 获得了1 289次余震修定后的震源位置。重新定位后, 余震基本沿昆仑山口-江错断裂呈NWW向线性分布, 震源深度由重新定位前主要集中于5~10km变为在5~15km深度范围内相对均匀地分布。根据重新定位后的余震分布特点并参考地质断层及现场考察的地震破裂带展布情况, 依据成丛地震发生在断层附近的原则, 选取了6个矩形区域内重新定位后的震源信息, 联合采用模拟退火与高斯-牛顿算法反演获得了每个区域断层面的详细参数。结果表明, 主干断裂为长约146km、总体走向为285°~290°的高倾角大型左旋走滑兼逆冲断裂。重新定位还显示, 主干断裂东、西两侧有分叉现象, 可能是大地震发生时期由于复杂的应力分配导致触发并新产生2条分支断裂, 断裂整体显示为树形。西侧分支走向为306°, 与主干断裂相交, 夹角为21°。东侧分支走向近EW, 与主干断裂的东段相连。

关键词: 双差定位, 玛多M7.4地震, 断层面解, 发震构造